what would you think accounts for a 50% better keeper rate of the T4i over a 50D?

I ask because its the same AF system and the T4i has a smaller View Finder

I did have a good 50D and was very pleased with it (was a substantial upgrade from my 30D). I hadn't considered a Rebel previously as the AF was just not quite there IME. When the T4i was released, I saw that the AF was up-specced and the framerate was improved to the min 5 FPS that I required (for what I like to shoot). So I figured I'd get one to throw a macro lens on at the very least, and to use as a second body at times.

Well it sure didn't take long to see that my keeper percentage was skyrocketing (BIFs especially). And the overall IQ was even better. I gave my 50D away after about a month! Getting used to shooting the Rebel was a breeze and I just loved the touchscreen. AF (speed) in LiveView was even passable. And I didn't even think twice about the smaller viewfinder. I was just as comfortable shooting the T4i as with any other (Canon) camera that I've owned/shot so far.

Now what I speculate regarding the improved AF is that even though the AF has the same specs as the older camera, I think that Canon has improved it and the AF algorithms as well. Kinda like two cars both being specced as 5-speed Manuals, but demonstrating a world of difference in actual performance. In any case, the camera just rocked, and I'd highly recommend it to anyone looking for an outstanding stills camera (you have to be willing to focus manually for Videos tho).

With that in mind I picked up a 70D for its new video autofocusing (with 18-135 STM), and for its deeper RAW buffer (did miss that for sports and BIFs in particular). I have admittedly been shooting some fairly static subjects so far with the 70D (in very low light mostly), and AF there has been above expectations. But I haven't gotten out with my 300 or 400 teles yet. That'll be the true test.

One note: the AF with the 100L Macro (at 1:1) has been simply outstanding with the 70D. It can now out-focus me! (I normally use the manual "sway" technique). It's now my preferred method when shooting macros in the wind! No joking.